1. Technical Field
The present invention relates to printers, copying machines and other image forming apparatuses using an intermediate transfer system.
2. Related Art
In recent years, for example, the following apparatus using an intermediate transfer system has been known as an image forming apparatus such as a printer using an electrophotographic system or an electrostatically recording system.
That is, in such an image forming apparatus, as shown in FIG. 13 by way of example, four image forming units 10Y, 100M, 100C and 100K for forming toner images of four colors, yellow (Y), magenta (M), cyan (C) and black (K) dedicatedly on image carriers 101 such as photoconductor drums are disposed in series. Toner images of yellow, magenta, cyan and black formed by the image forming units 100 (Y, M, C and K) respectively are temporarily (primarily) transferred to an endless intermediate transfer belt 201 which is rotating. The intermediate transfer belt 201 is disposed to be able to contact the image carriers 101 of the image forming units individually. Due to the rotation of the intermediate transfer belt 201, the toner images are conveyed to a secondary transfer position where a recording sheet 300 runs together. Then, the toner images are (secondarily) transferred to the recording sheet 300 in a lump, and fixed thereon. The reference numerals 150 in FIG. 13 represent primary transfer rolls for rotating the intermediate transfer belt 201 so as to press it against the image carriers 101 respectively, and transfer the images on the image carriers 101 to the intermediate transfer belt 201 respectively. The reference numerals 202-204 represent plural support rolls for stretching the intermediate transfer belt 201 so as to rotate and drive it.
According to this image forming apparatus, a full color (multi-color) image having a configuration where the aforementioned toner images of the four colors have been combined, or a single-color image such as a monochrome image composed of toner images of one or two colors of the aforementioned four colors is formed.
Of such image forming apparatuses, for example, there is an apparatus designed to operate at least a part of image carriers (for example, 101Y, 101M and 101C) of the plural image carriers 101 (Y, M, C and K) so that the rotation velocities thereof are changed over to velocities different from the rotation velocities of the other image carriers (for example, 101K) and the rotation velocity of the intermediate transfer belt 201. Thus, it is possible to obtain an image forming operation satisfying proper image forming conditions corresponding to a difference in kind of image to be formed (for example, a difference between a color image and a single-color image), a difference in kind of recording sheet or the like.
In a special example of the image forming apparatus, a color image forming mode and a monochrome image forming mode are prepared. When the monochrome image forming mode is selected, the rotation velocity of the image carrier 101K of the image forming unit 100K engaged in operation for forming a monochrome image is set to be equal to the rotation velocity of the intermediate transfer belt 201. On the other hand, the rotation velocities of the image carriers 101Y, 101M and 101C in the other image forming units 100Y, 100M and 100C which are not engaged in the operation for forming a monochrome image are changed over to be set to be lower than the rotation velocity of the image carrier 101K or the rotation velocity of the intermediate transfer belt 201. A monochrome image may be formed by operating (activating) the image forming apparatus in such changed conditions. When a monochrome image is formed in such conditions, the image carriers 101Y, 101M and 101C in the image forming units which are not engaged in the operation for forming the monochrome image can be prevented from being driven to rotate needlessly. Thus, the lives of the image carriers 101Y, 101M and 101C can be prevented from being shortened.